Network Working Group C. Jennings
Internet-Draft Cisco Systems
Expires: December 27, 2006 J. Peterson
NeuStar, Inc.
J. Fischl, Ed.
CounterPath Solutions, Inc.
June 25, 2006
Certificate Management Service for The Session Initiation Protocol (SIP)
draft-ietf-sip-certs-01
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This draft defines a Credential Service that allows Session
Initiation Protocol (SIP) User Agents (UAs) to use a SIP package to
discover the certificates of other users. This mechanism allows user
agents that want to contact a given Address-of-Record (AOR) to
retrieve that AOR's certificate by subscribing to the Credential
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Internet-Draft SIP Certificates June 20061. Introduction
SIP [6] provides a mechanism [18] for end-to-end encryption and
integrity using S/MIME [17]. Several security properties of SIP
depend on S/MIME, and yet it has not been widely deployed. One
reason is the complexity of providing a reasonable certificate
distribution infrastructure. This specification proposes a way to
address discovery, retrieval, and management of certificates for SIP
deployments. Combined with the SIP Identity [2] specification, this
specification allows users to have certificates that are not signed
by any well known certificate authority while still strongly binding
the user's identity to the certificate.
In addition, this specification provides a which mechanism allows SIP
User Agents such as IP phones to enroll and get their credentials
without any more configuration information than they commonly have
today. The end user expends no extra effort. It follows the Sacred
Framework RFC 3760 [7] for management of the credentials.
2. Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [5].
Certificate: A PKIX [13] style certificate containing a public key
and a list of identities in the SubjectAltName that are bound to
this key. The certificates discussed in this draft are generally
self signed and use the mechanisms in the SIP Identity [2]
specification to vouch for their validity. Certificates that are
signed by a certificate authority can also be used with all the
mechanisms in this draft, but it is expected that they are used
purely as a key carrier and that their validity is not checked.
Credential: For this document, credential means the combination of a
certificate and the associated private key.
password phrase: A password used to encrypt a PKCS#8 private key.
3. Overview
The general approach is to provide a new SIP service referred to as a
"credential service" that allows SIP User Agents (UAs) to subscribe
to other users' certificates using a new SIP event package [4]. The
certificate is delivered to the subscribing UA in a corresponding SIP
NOTIFY request. The identity of the certificate can be vouched for
using the Authentication Service from the SIP Identity [2]
specification, which uses the domain's certificate to sign the NOTIFY
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request. The credential service can manage public certificates as
well as the user's private keys. Users can update their credentials,
as stored on the credential service, using a SIP PUBLISH [3] request.
The UA authenticates to the credential service using a shared secret
when a UA is updating a credential. Typically the shared secret will
be the same one that is used by the UA to authenticate a REGISTER
request with the Registrar for the domain (usually with SIP Digest
Authentication).
The following figure shows Bob publishing his credentials from one of
his User Agents (e.g. his laptop software client), retrieving his
credentials from another of his User Agents (e.g. his mobile phone),
and then Alice retrieving Bob's certificate and sending a message to
Bob. SIP 200-class responses are omitted from the diagram to make the
figure easier to understand.
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credential server sends a NOTIFY that contains Bob's credentials.
The private key portion of the credential may have been encrypted
with a secret that only Bob's UA (and not the credential server)
knows. In this case, once Bob's UA decrypts the private key it will
be ready to go. Typically Bob's UA would do this when it first
registered on the network.
Some time later Alice decides that she wishes to discover Bob's
certificate so that she can send him an encrypted message or so that
she can verify the signature on a message from Bob. Alice's UA sends
a SUBSCRIBE message to Bob's AOR. The proxy in Bob's domain routes
this to the credential server via an authorization service. The
credential server returns a NOTIFY that contains Bob's public
certificate in the body. This is routed through an authentication
service that signs that this message really can validly claim to be
from the AOR "sip:bob@example.com". Alice's UA receives the
certificate and can use it to encrypt a message to Bob.
It is critical to understand that the only way that Alice can trust
that the certificate really is the one for Bob and that the NOTIFY
has not been spoofed is for Alice to check that the Identity [2]
header field value is correct.
The mechanism described in this document works for both self signed
certificates and certificates signed by well known certificate
authorities. However, most UAs would only use self signed
certificates and would use an Authentication Service as described in
[2] to provide a strong binding of an AOR to the certificates.
The mechanisms described in this draft allow for three different
styles of deployment:
1. Deployments where the credential server only stores certificates
and does not store any private key information. If the
deployment had users with multiple devices, some other scheme
(perhaps even manual provisioning) would be used to get the right
private keys onto all the devices that a user uses.
2. Deployments where the credential server stores certificates and
also stores an encrypted version of the private keys. The
credential server would not know or need the password phrase for
decrypting the private key. The credential server would help
move the private keys between devices but the user would need to
enter a password phrase on each device to allow that device to
decrypt (and encrypt) the private key information.
3. Deployments where the credential server stores the certificates
and private keys and also knows the password phrase for
decrypting the private keys. Deployments such as these may not
even use password phrases, in which case the private keys are not
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encrypted inside the PKCS#8 objects. This style of deployment
would often have the credential server, instead of the devices,
create the credentials.
4. UA Behavior with Certificates
When a User Agent wishes to discover some other user's certificate it
subscribes to the "certificate" SIP event package as described in
Section 6 to get the certificate. While the subscription is active,
if the certificate is updated, the Subscriber will receive the
updated certificate in a notification.
The Subscriber needs to decide how long it is willing to trust that
the certificate it receives is still valid. If the certificate is
revoked before it expires, the Notifier will send a notification with
an empty body to indicate that the certificate is no longer valid.
However, the Subscriber might not receive the notification if an
attacker blocks this traffic. The amount of time that the Subscriber
caches a certificate SHOULD be configurable. A default of one day is
RECOMMENDED.
Note that the actual duration of the subscription is unrelated to the
caching time or validity time of the corresponding certificate.
Allowing subscriptions to persist after a certificate is no longer
valid ensures that Subscribers receive the replacement certificate in
a timely fashion. In some cases, the Notifier will not allow
unauthenticated subscriptions to persist. The Notifier could return
an immediate notification with the certificate in response to
subscribe and then immediately terminate subscription, setting the
reason parameter to "probation". The Subscriber will have to
periodically poll the Notifier to verify validity of the certificate.
If the UA uses a cached certificate in a request and receives a 437
(Unsupported Certificate) response, it SHOULD remove the certificate
it used from the cache, attempt to fetch the certificate again. If
the certificate is changed, then the UA SHOULD retry the original
request again with the new certificate. This situation usually
indicates that the certificate was recently updated, and that the
Subscriber has not received a corresponding notification. If the
certificate fetched is the same as the one that was previously in the
cache, then the UA SHOULD NOT try the request again. This situation
can happened when the request was retargeted to a different user than
the original request. The 437 response is defined in [2].
Note: A UA that has a presence list MAY want to subscribe to the
certificates of all the presentities in the list when the UA
subscribes to their presence, so that when the user wishes to
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contact a presentity, the UA will already have the appropriate
certificate. Future specifications might consider the possibility
of retrieving the certificates along with the presence documents.
The details of how a UA deals with receiving encrypted messages is
outside the scope of this specification. It is worth noting that if
Charlie's UAS receives a request that is encrypted to Bob, it would
be valid and legal for that UA to send a 302 redirecting the call to
Charlie.
5. UA Behavior with Credentials
UAs discover their own credentials by subscribing to their AOR with
an event type of credential as described in Section 7. After a UA
registers, it SHOULD retrieve its credentials by subscribing to them
as described in Section 6.6.
When a UA discovers its credential, the private key information might
be encrypted with a password phrase. The UA SHOULD request that the
user enter the password phrase on the device, and the UA MAY cache
this password phrase for future use.
There are several different cases in which a UA should generate a new
credential:
o If the UA receives a NOTIFY with no body for the credential
package.
o If the certificate has expired.
o If the certificate's notAfter date is within the next 600 seconds,
the UA SHOULD attempt to create replacement credentials. The UA
does this by waiting a random amount of time between 0 and 300
seconds. If no new credentials have been received in that time,
the UA creates new credentials to replace the expiring ones and
sends them in a PUBLISH request (with a SIP-If-Match header set to
the current etag). This makes credential collisions both unlikely
and harmless.
o If the user of the device has indicated via the user interface
that they wish to revoke the current certificate and issue a new
one.
Credentials are created by creating a new key pair which will require
appropriate randomness, and then creating a certificate as described
in Section 9.6. The UA MAY encrypt the private key with a password
phrase supplied by the user. Next, the UA updates the user's
credential by sending a PUBLISH [3] request with the credentials or
just the certificate as described in Section 7.9.
If a UA wishes to revoke the existing certificate without publishing
a new one, it MUST send a PUBLISH with an empty body to the
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credential server.
6. Event Package Formal Definition for "certificate"6.1. Event Package Name
This document defines a SIP Event Package as defined in RFC 3265 [4].
The event-package token name for this package is:
certificate
6.2. Event Package Parameters
This package defines the "etag" Event header parameter which is valid
only in NOTIFY requests. It contains a token which represents the
SIP etag value at the time the notification was sent. Considering
how infrequently credentials are updated, this hint is very likely to
be the correct etag to use in the SIP-If-Match header in a SIP
PUBLISH request to update the current credentials.
6.3. SUBSCRIBE Bodies
This package does not define any SUBSCRIBE bodies.
6.4. Subscription Duration
Subscriptions to this event package can range from no time to weeks.
Subscriptions in days are more typical and are RECOMMENDED. The
default subscription duration for this event package is one day.
The credential service is encouraged to keep the subscriptions active
for AORs that are communicating frequently, but the credential
service MAY terminate the subscription at any point in time.
6.5. NOTIFY Bodies
The body of a NOTIFY request for this package MUST either be empty or
contain an application/pkix-cert body (as defined in [10]) that
contains the certificate, unless an Accept header has negotiated some
other type. The Content-Disposition MUST be set to "signal", as
defined in as defined in [16].
A future extension MAY define other NOTIFY bodies. If no "Accept"
header is present in the SUBSCRIBE, the body type defined in this
document MUST be assumed.
Implementations which generate large notifications are reminded to
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follow the message size restrictions for unreliable transports
articulated in Section 18.1.1 of SIP.
6.6. Subscriber Generation of SUBSCRIBE Requests
A UA discovers a certificate by sending a SUBSCRIBE request with an
event type of "certificate" to the AOR for which a certificate is
desired. In general, the UA stays subscribed to the certificate for
as long as it plans to use and cache the certificate, so that the UA
can be notified about changes or revocations to the certificate.
Subscriber User Agents will typically subscribe to certificate
information for a period of hours or days, and automatically attempt
to re-subscribe just before the subscription is completely expired.
When a user de-registers from a device (logoff, power down of a
mobile device, etc.), subscribers SHOULD unsubscribe by sending a
SUBSCRIBE request with an Expires header of zero.
6.7. Notifier Processing of SUBSCRIBE Requests
When a SIP credential server receives a SUBSCRIBE request with the
certificate event-type, it is not necessary to authenticate the
subscription request. The Notifier MAY limit the duration of the
subscription to an administrator-defined period of time. The
duration of the subscription does not correspond in any way to the
period for which the certificate will be valid.
When the credential server receives a SUBSCRIBE request for a
certificate, it first checks to see if it has credentials for the
requested URI. If it does not have a certificate, it returns a
NOTIFY request with an empty message body.
6.8. Notifier Generation of NOTIFY Requests
Immediately after a subscription is accepted, the Notifier MUST send
a NOTIFY with the current certificate, or an empty body if no
certificate is available for the target user. In either case it
forms a NOTIFY with the From header field value set to the value of
the To header field in the SUBSCRIBE request. This server sending
the NOTIFY needs either to implement an Authentication Service (as
described in SIP Identity [2]) or else the server needs to be set up
such that the NOTIFY request will be sent through an Authentication
Service. Sending the NOTIFY request through the Authentication
Service requires the SUBSCRIBE request to have been routed through
the Authentication Service, since the NOTIFY is sent within the
dialog formed by the subscription.
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The resulting NOTIFY will contain an application/pkix-cert body that
contains the requested certificate. The UA MUST follow the
procedures in Section 9.3 to decide if the received certificate can
be used. The UA needs to cache this certificate for future use. The
maximum length of time it should be cached for is discussed in
Section 9.1. The certificate MUST be removed from the cache if the
certificate has been revoked (if a NOTIFY with an empty body is
received), or if it is updated by a subsequent NOTIFY. The UA MUST
check that the NOTIFY is correctly signed by an Authentication
Service as described in [2]. If the identity asserted by the
Authentication Service does not match the AOR that the UA subscribed
to, the certificate in the NOTIFY is discarded and MUST NOT be used.
6.10. Handling of Forked Requests
This event package does not permit forked requests. At most one
subscription to this event type is permitted per resource.
6.11. Rate of Notifications
Notifiers SHOULD NOT generate NOTIFY requests more frequently than
once per minute.
6.12. State Agents and Lists
Implementers MUST NOT implement state agents for this event type.
Likewise, implementations MUST NOT use the event list extension [19]
with this event type. It is not possible to make such an approach
work, because the Authentication service would have to simultaneously
assert several different identities.
6.13. Behavior of a Proxy Server
There are no additional requirements on a SIP Proxy, other than to
transparently forward the SUBSCRIBE and NOTIFY requests as required
in SIP. This specification describes the Proxy, Authentication
service, and credential service as three separate services, but it is
certainly possible to build a single SIP network element that
performs all of these services at the same time.
7. Event Package Formal Definition for "credential"7.1. Event Package Name
This document defines a SIP Event Package as defined in RFC 3265 [4].
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The event-package token name for this package is:
credential
7.2. Event Package Parameters
This package defines the "etag" Event header parameter which is valid
only in NOTIFY requests. It contains a token which represents the
SIP etag value at the time the notification was sent. Considering
how infrequently credentials are updated, this hint is very likely to
be the correct etag to use in the SIP-If-Match header in a SIP
PUBLISH request to update the current credentials.
etag-param = "etag" EQUAL token
7.3. SUBSCRIBE Bodies
This package does not define any SUBSCRIBE bodies.
7.4. Subscription Duration
Subscriptions to this event package can range from hours to one week.
Subscriptions in days are more typical and are RECOMMENDED. The
default subscription duration for this event package is one day.
The credential service SHOULD keep subscriptions active for UAs that
are currently registered.
7.5. NOTIFY Bodies
The NOTIFY MUST contain a multipart/mixed (see [14]) body that
contains both an application/pkix-cert body with the certificate and
an application/pkcs8 body that has the associated private key
information for the certificate. The Content-Disposition MUST be set
to "signal" as defined in [16].
A future extension MAY define other NOTIFY bodies. If no "Accept"
header is present in the SUBSCRIBE, the body type defined in this
document MUST be assumed.
The application/pkix-cert body is a DER encoded X.509v3 certificate
[10]. The application/pkcs8 body contains a DER-encoded PKCS#8 [1]
object that contains the private key. The PKCS#8 objects MUST be of
type PrivateKeyInfo. The integrity and confidentiality of the PKCS#8
objects is provided by the TLS transport. The transport encoding of
all the MIME bodies is binary.
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A Subscriber User Agent will subscribe to its credential information
for a period of hours or days and will automatically attempt to re-
subscribe before the subscription has completely expired.
The Subscriber SHOULD subscribe to its credentials whenever a new
user becomes associated with the device (a new login). The
subscriber SHOULD also renew its subscription immediately after a
reboot, or when the subscriber's network connectivity has just been
re-established.
The UA needs to authenticate with the credential service for these
operations. The UA MUST use TLS to connect to the server. The UA
may be configured with a specific name for the credential service;
otherwise normal SIP routing is used. As described in RFC 3261, the
TLS connection needs to present a certificate that matches the
expected name of the server to which the connection was formed, so
that the UA knows it is talking to the correct server. Failing to do
this may result in the UA publishing its private key information to
an attacker. The credential service will authenticate the UA using
the usual SIP Digest mechanism, so the UA can expect to receive a SIP
challenge to the SUBSCRIBE or PUBLISH requests.
7.7. Notifier Processing of SUBSCRIBE Requests
When a credential service receives a SUBSCRIBE for a credential, the
credential service has to authenticate and authorize the UA and
validate that adequate transport security is being used. Only a UA
that can authenticate as being able to register as the AOR is
authorized to receive the credentials for that AOR. The credential
Service MUST digest challenge the UA to authenticate the UA and then
decide if it is authorized to receive the credentials. If
authentication is successful, the Notifier MAY limit the duration of
the subscription to an administrator-defined period of time. The
duration of the subscription MUST not be larger than the length of
time for which the certificate is still valid. The Expires header
SHOULD be set so that it is not longer than the notAfter date in the
certificate.
7.8. Notifier Generation of NOTIFY Requests
Once the UA has authenticated with the credential service and the
subscription is accepted, the credential service MUST immediately
send a Notify request. The Notifier SHOULD include the current etag
value in the "etag" Event package parameter in the NOTIFY request.
The Authentication Service is applied to this NOTIFY request in the
same way as the certificate subscriptions. If the credential is
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revoked, the credential service MUST terminate any current
subscriptions and force the UA to re-authenticate by sending a NOTIFY
with its Subscription-State header set to "terminated" and a reason
parameter of "deactivated". (This causes a Subscriber to retry the
subscription immediately.) This is so that if a secret for
retrieving the credentials gets compromised, the rogue UA will not
continue to receive credentials after the compromised secret has been
changed.
Any time the credentials for this URI change, the credential service
MUST send a new NOTIFY to any active subscriptions with the new
credentials.
7.9. Generation of PUBLISH Requests
A user agent SHOULD be configurable to control whether it publishes
the credential for a user or just the user's certificate.
When publishing just a certificate, the body contains an application/
pkix-cert. When publishing a credential, the body contains a
multipart/mixed containing both an application/pkix-cert and an
application/pkcs8 body.
When the UA sends the PUBLISH [3] request, it needs to do the
following:
o The Expires header field value in the PUBLISH request SHOULD be
set to match the time for which the certificate is valid.
o If the certificate includes Basic Constraints, it SHOULD set the
CA flag to false.
o The PUBLISH request SHOULD include a SIP-If-Match header field
with the previous etag from the subscription. This prevents
multiple User Agents for the same AOR from publishing conflicting
credentials. Note that UAs replace credentials that are about to
expire at a random time (described in Section 5), reducing the
chance of publishing conflicting credentials even without using
the etag.
7.10. Notifier Processing of PUBLISH Requests
When the credential service receives a PUBLISH to update credentials,
it MUST authenticate and authorize this request the same way as for
subscriptions for credentials. If the authorization succeeds, then
the credential service MUST perform the following check on the
certificate:
o One of the names in the SubjectAltName of the certificate matches
the authorized user making the request.
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o The notBefore validity time MUST NOT be in the future.
o The notAfter validity time MUST be in the future.
o If a CA Basic Constraint is set in the certificate, it is set to
false.
If all of these succeed, the credential service updates the
credential for this URI, processes all the active certificates and
credential subscriptions to this URI, and generates a NOTIFY request
with the new credential or certificate.
If the Subscriber submits a PUBLISH request with no body, this
revokes the current credentials and causes all subscriptions to the
credential package to be deactivated as described in the previous
section. (Note that subscriptions to the certificate package are NOT
terminated; each subscriber to the certificate package receives a
notification with an empty body.)
7.11. Subscriber Processing of NOTIFY Requests
When the UA receives a valid NOTIFY request, it should replace its
existing credentials with the new received ones. If the UA cannot
decrypt the PKCS#8 object, it MUST send a 437 (Unsupported
Certificate) response. Later if the user provides a new password
phrase for the private key, the UA can subscribe to the credentials
again and attempt to decrypt with the new password phrase.
7.12. Handling of Forked Requests
This event package does not permit forked requests.
7.13. Rate of Notifications
Notifiers SHOULD NOT generate NOTIFY requests more frequently than
once per minute.
7.14. State Agents and Lists
Implementers MUST NOT implement state agents for this event type.
Likewise, implementations MUST NOT use the event list extension [19]
with this event type.
7.15. Behavior of a Proxy Server
The behavior is identical to behavior described for certificate
subscriptions described in Section 6.13.
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When Alice's UA wishes to publish Alice's public and private keys to
the credential service, it sends a PUBLISH request like the one
below. This must be sent over a TLS connection in which the other
end of the connection presents a certificate that matches the
credential service for Alice and digest challenges the request to
authenticate her.
PUBLISH sips:alice@atlanta.example.com SIP/2.0
...
Content-Type: multipart/mixed;boundary=boundary
Content-Disposition: signal
--boundary
Content-ID: 123
Content-Type: application/pkix-cert
< Public certificate for Alice >
--boundary
Content-ID: 456
Content-Type: application/pkcs8
< Private Key for Alice >
--boundary
If one of Alice's UAs subscribes to the credential event, the UA will
be digest challenged, and the NOTIFY will include a body similar to
the one in the PUBLISH section above.
9. Security Considerations
The high level message flow from a security point of view is
summarized in the following figure. The 200 responses are removed
from the figure as they do not have much to do with the overall
security.
In this figure, authC refers to authentication and authZ refers to
authorization.
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Alice Server Bob UA
| | TLS Handshake | 1) Client authC/Z server
| |<---------------->|
| | PUBLISH | 2) Client sends request
| |<-----------------| (write credential)
| | Digest Challenge | 3) Server challenges client
| |----------------->|
| | PUBLISH + Digest | 4) Server authC/Z client
| |<-----------------|
| | time... |
| | |
| | TLS Handshake | 5) Client authC/Z server
| |<---------------->|
| | SUBSCRIBE | 6) Client sends request
| |<-----------------| (read credential)
| | Digest Challenge | 7) Server challenges client
| |----------------->|
| | SUBSCRIBE+Digest | 8) Server authC/Z client
| |<-----------------|
| | NOTIFY | 9) Server returns credential
| |----------------->|
| |
| SUBSCRIBE | 10) Client requests certificate
|---------->|
| |
|NOTIFY+AUTH| 11) Server returns user's certificate and signs that
|<----------| it is valid using certificate for the domain
| |
When the UA, labeled Bob, first created a credential for Bob, it
would store this on the credential server. The UA authenticated the
Server using the certificates from the TLS handshake. The Server
authenticated the UA using a digest style challenge with a shared
secret.
The UA, labeled Bob, wishes to request its credentials from the
server. First it forms a TLS connection to the Server, which
provides integrity and privacy protection and also authenticates the
server to Bob's UA. Next the UA requests its credentials using a
SUBSCRIBE request. The Server digest challenges this to authenticate
Bob's UA. The server and Bob's UA have a shared secret that is used
for this. If the authentication is successful, the server sends the
credentials to Bob's UA. The private key in the credentials may have
been encrypted using a shared secret that the server does not know.
A similar process would be used for Bob's UA to publish new
credentials to the server. Bob's UA would send a PUBLISH request
containing the new credentials. When this happened, all the other
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UAs that were subscribed to Bob's credentials would receive a NOTIFY
with the new credentials.
Alice wishes to find Bob's certificate and sends a SUBSCRIBE to the
server. The server sends the response in a NOTIFY. This does not
need to be sent over a privacy or integrity protected channel, as the
Authentication service described in [2] provides integrity protection
of this information and signs it with the certificate for the domain.
This whole scheme is highly dependent on trusting the operators of
the credential service and trusting that the credential service will
not be compromised. The security of all the users will be
compromised if the credential service is compromised.
Note: There has been significant discussion of the topic of
avoiding deployments in which the credential servers store the
private keys, even in some encrypted form that the credential
server does not know how to decrypt. Various schemes were
considered to avoid this but they all result in either moving the
problem to some other server, which does not seem to make the
problem any better, or having a different credential for each
device. For some deployments where each user has only one device
this is fine but for deployments with multiple devices, it would
require that when Alice went to contact Bob, Alice would have to
provide messages encrypted for all of Bob's devices. The sipping
working group did consider this architecture and decided it was
not appropriate due both to the information it revealed about the
devices and users and the amount of signaling required to make it
work.
This specification requires that TLS be used for the SIP
communications to place and retrieve a UA's private key. This
provides security in two ways:
1. Confidentiality is provided for the digest authentication
exchange, thus protecting it from dictionary attacks.
2. Confidentiality is provided for the private key, thus protecting
it from being exposed to passive attackers.
In order to prevent man-in-the-middle attacks, TLS clients MUST check
that the SubjectAltName of the certificate for the server they
connected to exactly matches the server they were trying to connect
to. Failing to use TLS or selecting a poor cipher suite (such as
NULL encryption) may result in credentials, including private keys,
being sent unencrypted over the network and will render the whole
system useless.
The correct checking of chained certificates as specified in TLS [11]
is critical for the client to authenticate the server. If the client
does not authenticate that it is talking to the correct credential
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service, a man in the middle attack is possible.
9.1. Certificate Revocation
If a particular credential needs to be revoked, the new credential is
simply published to the credential service. Every device with a copy
of the old credential or certificate in its cache will have a
subscription and will rapidly (order of seconds) be notified and
replace its cache. Clients that are not subscribed will subscribe
when they next need to use the certificate and will get the new
certificate.
It is possible that an attacker could mount a DOS attack such that
the UA that had cached a certificate did not receive the NOTIFY with
its revocation. To protect against this attack, the UA needs to
limit how long it caches certificates. After this time, the UA would
invalidate the cached information even though no NOTIFY had ever been
received due to the attacker blocking it.
The duration of this cached information is in some ways similar to a
device deciding how often to check a CRL list. For many
applications, a default time of 1 day is suggested, but for some
applications it may be desirable to set the time to zero so that no
certificates are cached at all and the credential is checked for
validity every time the certificate is used.
9.2. Certificate Replacement
The UAs in the system replace the certificates close to the time that
the certificates would expire. If a UA has used the same key pair to
encrypt a very large volume of traffic, the UA MAY choose to replace
the credential with a new one before the normal expiration.
9.3. Trusting the Identity of a Certificate
When a UA wishes to discover the certificate for
sip:alice@example.com, the UA subscribes to the certificate for
alice@example.com and receives a certificate in the body of a SIP
NOTIFY request. The term original URI is used to describe the URI
that was in the To header field value of the SUBSCRIBE request. So
in this case the original URI would be sip:alice@example.com.
If the certificate is signed by a trusted CA, and one of the names in
the SubjectAltName matches the original URI, then this certificate
MAY be used but only for exactly the original URI and not for other
identities found in the SubjectAltName. Otherwise, there are several
steps the UA MUST perform before using this certificate.
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o The From header in the NOTIFY request MUST match the original URI
that was subscribed to.
o The UA MUST check the Identity header as described in the Identity
[2] specification to validate that bodies have not been tampered
with and that an Authentication Service has validated this From
header.
o The UA MUST check the validity time of the certificate and stop
using the certificate if it is invalid. (Implementations are
reminded to verify both the notBefore and notAfter validity
times.)
o The certificate MAY have several names in the SubjectAltName but
the UA MUST only use this certificate when it needs the
certificate for the identity asserted by the Authentication
Service in the NOTIFY. This means that the certificate should
only be indexed in the certificate cache by the AOR that the
Authentication Service asserted and not by the value of all the
identities found in the SubjectAltName list.
These steps result in a chain of bindings that result in a trusted
binding between the original AOR that was subscribed to and a public
key. The original AOR is forced to match the From. The
Authentication Service validates that this request did come from the
identity claimed in the From header field value and that the bodies
in the request that carry the certificate have not been tampered
with. The certificate in the body contains the public key for the
identity. Only the UA that can authenticate as this AOR, or devices
with access to the private key of the domain, can tamper with this
body. This stops other users from being able to provide a false
public key. This chain of assertion from original URI, to From, to
body, to public key is critical to the security of the mechanism
described in this specification. If any of the steps above are not
followed, this chain of security will be broken and the system will
not work.
9.4. Conformity to the SACRED Framework
This specification uses the security design outlined in the SACRED
Framework [7]. Specifically, it follows the cTLS architecture
described in section 4.2.2 of RFC 3760. The client authenticates the
server using the server's TLS certificate. The server authenticates
the client using a SIP digest transaction inside the TLS session.
The TLS sessions form a strong session key that is used to protect
the credentials being exchanged.
9.5. Crypto Profiles
Credential services SHOULD implement the server name indication
extensions in RFC 3546 [8] and they MUST support a TLS profile of
TLS_RSA_WITH_AES_128_CBC_SHA as described in RFC 3268 [9] as a
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profile of TLS_RSA_WITH_3DES_EDE_CBC_SHA.
The PKCS#8 in the clients MUST implement PBES2 with a key derivation
algorithm of PBKDF2 using HMAC with SHA1 and an encryption algorithm
of DES-EDE2-CBC-Pad as defined in RFC 2898 [12]. It is RECOMMENDED
that this profile be used when using PKCS#8. A different passphrase
SHOULD be used for the PKCS#8 encryption than is used for server
authentication.
9.6. User Certificate Generation
The certificates should be consistent with RFC 3280 [13]. A
signatureAlgorithm of sha1WithRSAEncryption MUST be implemented. The
Issuers SHOULD be the same as the subject. Given the ease of issuing
new certificates with this system, the Validity can be relatively
short. A Validity of one year or less is RECOMMENDED. The
subjectAltName must have a URI type that is set to the SIP URL
corresponding to the user AOR. It MAY be desirable to put some
randomness into the length of time for which the certificates are
valid so that it does not become necessary to renew all the
certificates in the system at the same time.
It is worth noting that a UA can discover the current time by looking
at the Date header field value in the 200 response to a REGISTER
request.
9.7. Compromised Authentication Service
One of this worst attacks against this system would be if the
Authentication Service were compromised. This attack is somewhat
analogous to a CA being compromised in traditional PKI systems. The
attacker could make a fake certificate for which it knows the private
key, use it to receive any traffic for a given use, and then re-
encrypt that traffic with the correct key and forward the
communication to the intended receiver. The attacker would thus
become a man in the middle in the communications.
There is not too much that can be done to protect against this. A UA
MAY subscribe to its own certificate under some other identity to try
to detect whether the credential server is handing out the correct
certificates. It will be difficult to do this in a way that does not
allow the credential server to recognize the user's UA.
The UA MAY also save the fingerprints of the cached certificates and
warn users when the certificates change significantly before their
expiry date.
The UA MAY also allow the user to see the fingerprints for the cached
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certificates so that they can be verified by some other out of band
means.
10. IANA Considerations
This specification defines two new event packages that IANA is
requested to add the registry at:
http://www.iana.org/assignments/sip-events
It also defines a new mime type that IANA is requested to add to the
registry at:
http://www.iana.org/assignments/media-types/application10.1. Certificate Event Package
To: ietf-sip-events@iana.org
Subject: Registration of new SIP event package
Package Name: certificate
Is this registration for a Template Package: No
Published Specification(s): This document
New Event header parameters: This package defines no
new parameters
Person & email address to contact for further information:
Cullen Jennings <fluffy@cisco.com>
10.2. Credential Event Package
To: ietf-sip-events@iana.org
Subject: Registration of new SIP event package
Package Name: credential
Is this registration for a Template Package: No
Published Specification(s): This document
New Event header parameters: "etag"
Person & email address to contact for further information:
Cullen Jennings <fluffy@cisco.com>
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